EFFECT OF CORROSION ON CONCRETE REINFORCEMENT MECHANICAL AND PHYSICAL-2

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EFFECT OF CORROSION ON CONCRETE REINFORCEMENT MECHANICAL AND PHYSICAL-2 Powered By Docstoc
					   International Journal of Civil Engineering and CIVIL ENGINEERING AND
   INTERNATIONAL JOURNAL OF Technology (IJCIET), ISSN 0976 – 6308
   (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME
                             TECHNOLOGY (IJCIET)

ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)                                                         IJCIET
Volume 4, Issue 3, May - June (2013), pp. 176-184
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           EFFECT OF CORROSION ON CONCRETE REINFORCEMENT
                 MECHANICAL AND PHYSICAL PROPERTIES

                                     Sanad A.M.1 and Hassan H.A.2
              (Construction & Building Department, College of Engineering & Technology,
              Arab Academy for Science, Technology & Maritime Transport, Cairo, Egypt)



      ABSTRACT

              Corrosion of concrete reinforcement is a major factor affecting the deterioration of
      RC structures. During corrosion, steel undergoes several phases of chemical reactions with
      consequent variation in steel section geometry and mechanical properties. At ultimate
      corrosion stage, the effective cross section area of steel is reduced with equivalent decrease in
      load carrying capacity leading to unsafe structures. During initial phase of corrosion,
      chemical reactions generate new products which irregularly increase bars’ diameters. The
      resulted products induce additional stresses on the structural member, causing cracking and
      spalling of the concrete cover. Corrosion cracking increases further corrosion rate by loss of
      protective cover and direct exposure to corrosive environment. A comprehensive
      experimental program was conducted to evaluate the effect of several degrees of corrosion on
      the mechanical and physical properties of concrete reinforcement bars. Three types of carbon
      steel bars were used: plain, deformed, and epoxy-coated deformed bars. The results showed
      clearly the effect of corrosion on increasing the rate of deterioration of RC members’
      strength.

      Keywords: Steel corrosion, Reinforced Concrete Deterioration, Mechanical Properties.

 1.          INTRODUCTION

             Recently the aspects of concrete durability and performance have become a major
      subject of discussion especially when the concrete is subjected to severe environment.
      Corrosion of steel bars is the main factor influencing both the concrete durability and strength
      [1]. The corrosion products of the steel reinforcement can expand four to five times its
      original volume, developing high pressures within the concrete, which cause cracking and
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     (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

     spalling of the concrete cover and expose the rebar to further corrosion activity. The potential
     consequences of the corrosion problem can be summed up in the continuous reduction in
     strength, stiffness, durability and designed life time of concrete structural elements reinforced
     with conventional steel. According to the ASTM [2], corrosion is defined as “the chemical or
     electrochemical reaction between a material, usually a metal, and its environment that
     produces a deterioration of the material and its properties”. Corrosion reduces the ribs height
     of the deformed bar which causes reduction in the contact area between the ribs and the
     concrete leading to reduction in the bond strength. Corrosion may also affect the rib face
     angle in the advanced stages; moreover, ribs of deformed bars are eventually lost at high level
     of corrosion. Corrosion of reinforced bars is usually associated with the increase of the crack
     width [3]. The increase of the corrosion products around the bar leads to increase of bursting
     force and tension cracking of the surrounding concrete, as the corrosion increases, the crack
     width becomes wider and the bond strength decreases [4].
             Rapid deterioration of reinforced concrete buildings in Alexandria, Egypt has become
     a major problem for sea front buildings’ dwellers, where the disaster exceeds the value of
     money and extends to human lives. In the last decade many reinforced concrete buildings
     collapsed with a majority located in coastal cities. Over eighty percent of these collapses
     were at Alexandria and Damietta which are located at the north coast of Egypt facing the salt
     attack of the Mediterranean Sea. This high percentage highlights the importance of
     investigating the common factors that lead to the collapse of those buildings [1].

2.          RESEARCH SCOPE AND OBJECTIVES

             The main objective of this paper is to study the rate of reinforcement corrosion for
     three different types of steel embedded in four types of concrete. The effect of corrosion on
     the mechanical & physical properties of the embedded reinforcement is also investigated and
     the study is conducted at four phases of corrosion; un-corroded bars, pre-cracking, cracking
     and severely corroded bars. A comprehensive experimental program was implemented to
     identify the effect of four parameters; the water/cement ratio, concrete strength, type of steel
     reinforcement and coating material. The tested mechanical properties included the loss of the
     tensile strength and steel bars’ ductility; where, the measurement of physical properties
     included the mass and rib height loss of the bars.

3.          EXPERIMENTAL PROGRAM

             The effect of steel corrosion on the durability of seafront reinforced concrete
     structures was investigated experimentally at AASTMT Labs where the four types of
     concretes considered had variable water/cement ratios and variable ultimate strengths as
     shown in Table 1. The effect of corrosion in regular carbon steel bars was evaluated for Plain
     bars, un-coated deformed bars and epoxy coated deformed bars. The bars diameter was
     16mm and used to reinforce a 100mm diameter by 200mm height concrete cylinder. The
     specimen was reinforced with a single bar located in the center as shown in Fig. 1.




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(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

                          Table 1: Test results for concrete specimens

                                                                              Average Stress of
 Mixture    Concrete Density     Slump
                                                       Test Type             three samples (after
   Type         (kN/m3)              (mm)
                                                                               56 days) (MPa)

 30MPa,                                          Compressive Strength               33.74
                  23.1                29
 w/c=0.32                                       Splitting Tensile Strength          3.69

 44MPa,                                          Compressive Strength               48.58
                  23.3                30
 w/c=0.32                                       Splitting Tensile Strength          4.60

 60MPa,                                          Compressive Strength               62.90
                  23.3                33
 w/c=0.32                                       Splitting Tensile Strength          4.40

 44MPa,                                          Compressive Strength               49.88
                  24.2                32
 w/c=0.52                                       Splitting Tensile Strength          3.98




                         Figure 1.         cylinderical concrete specimens


       Different dosages of super plasticizers were added to the mixtures having low water
cement ratios (w/c=0.32) to obtain approximately the same slump range as the 44MPa
mixture with high water cement ratio (w/c=0.52). This technique was used to investigate
separately the effect of concrete strength and the effect of water cement ratio without any
                              fect
other variable that might affect the mechanical properties of the concrete. The tested
mechanical and physical properties of the used steel bars before corrosion initiation are
shown in Table 2.




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                  Table 2: Tested mechanical and physical properties of steel bars
                               16mm un coated         16mm un coated       16mm epoxy coated
                                plain steel bars     deformed steel bars   deformed steel bars
      Steel Grade                    24/35                 40/60                 40/60
      Code                          PL-UC
                                    PL                    DF-UC                  DF-C
                                                                                 DF
      Effective diameter               16                  15.75                 15.85
      Yield stress (MPa)              250                  564.5                 546.5
      Tensile strength (MPa)          360                   676                   658
      Elongation percentage           21.9                   13                   12.5

4.         ACCELERATED CORROSION SET-UP

         Accelerated corrosion tests are used to obtain qualitative information on corrosion
 behavior in a relative short period compared to the field corrosion test. Accelerated corrosion
 tests have been used successfully to determine the susceptibility of the reinforcing and other
 forms of structural steel to localized attacks such as pitting corrosion, stress corrosion and
 other forms of corrosion [5]. Before testing, all concrete samples were set for curing for 2
 months and specimens tested at end of this period were defined as zero corrosion samples and
 served as the control specimens. The rest of specimens were placed in fiber tank with
 dimension 165x85 cm containing an electrolytic solution [5% sodium chloride NaCl by the
 weight of water]. A steel mesh was placed in the bottom of tank to carry the specimens and
 connected to 12V power supply through a single steel bar as shown schematically in Fig. 2.
                                         arranged
 The direction of electrical current was arranged so that the single steel bar served as cathode,
 while the specimens’ bars served as anodes.
                                                                              Pre cracking
         Based on the crack width; three corrosion phases were defined; Pre-cracking stage
 considered when the electrical current measurement started to increase but before any crack
 was visible. Cracking stage considered when the first crack appeared on the concrete
 specimen regardless its width, and severe corrosion stage considered when any crack width
                                                                          all
 reached 4mm. The accelerated corrosion test was terminated when all stages of corrosion
 took place for all steel types.




             Figure 2: schematic diagram showing the accelerated corrosion set
                        chematic                                           set-up

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     International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
     (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

5.                             EXPERIMENTAL PROGRAM RESULTS

             The results of this research are divided into two parts; the first concerning the effect of
     corrosion on the mechanical properties of the reinforcing bars including the yield stress, the
     tensile strength and the ductility. The second part shows the effect of corrosion on the
     physical properties including the mass, the rib height, and the cross-sectional area of the bars.

5.1           Effect of Corrosion on the Bars’ Mechanical Properties
             The corroded bars were removed from the samples, after performing a pull-out test
     for concrete-reinforcement bond strength. Then, theywere subjected to axial tensile test to
     study the effect of corrosion on the tensile strength and ductility of the bars. The stress-strain
     curves were plotted for each concrete and steel type at different degrees of corrosion. Figs. 3
     to 5 show an example of stress-strain behaviors for the bars embedded in the 44MPa, 0.52
     water/cement ratio concrete cylinders.The tensile stress was calculated by dividing the tensile
     load by the average cross sectional area of bar, taken as the average between the area of the
     corroded embedded part and the un-corroded area of the protruding part. The strain was
     calculated by dividing the extension values taken from the machine LVDT by the bar initial
     length.From the figures, it can be seen that corrosion affects the steel mechanical properties
     negatively. At zero corrosion stage, all bars showed large ductility and high yield stress.
     However, these ductility regions start to disappear as the corrosion propagated from pre-
     cracking to severe corrosion stage and all bars failed at lower extensions due to large decrease
     in ductility.Nearly all steel grades showed similarreduction in ductility values.


                              400

                              350

                              300
       Tensile stress (Mpa)




                              250

                              200

                              150

                              100                                                           Zero corrosion
                                                                                            Pre-cracking
                               50
                                                                                            Cracking
                                0                                                           Severe corrosion
                                    0      0.05            0.1             0.15            0.2             0.25
                                                            Strain (mm/mm)

                                        Figure 3: stress-strain curve for un-coated plain bars




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 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME


                                800

                                700
        Tensile stress (Mpa)




                                600

                                500

                                400

                                300

                                200                                                                               Zero corrosion
                                                                                                                  Pre-cracking
                                100
                                                                                                                  Cracking
                                     0                                                                            Severe corrosion
                                             0      0.02          0.04      0.06       0.08          0.1              0.12           0.14
                                                                            Strain (mm/mm)

                                                   Figure 4: stress-strain curve for un-coated deformed bars


                               700

                               600
 Tensile stress (Mpa)




                               500

                               400

                               300

                               200                                                                                Zero corrosion
                                                                                                                  Pre-cracking
                               100                                                                                Cracking
                                                                                                                  Severe corrosion
                                0
                                         0       0.01      0.02     0.03    0.04     0.05     0.06         0.07        0.08      0.09
                                                                           Strain (mm/mm)

                                                 Figure 5: Stress-strain curve for epoxy-coated deformed bars


         A clear reduction in yield stress and ultimate stress is also observed in all specimens.
As corrosion propagates, the safety factor used in the designing process is drastically reduced
due to the decrease in yield stress and the over-all advantage of structural ductility also
disappears, leading to sudden failure without signs of large deformation in the structure.
Corrosion of steel over whelms its advantages when the effective cross section is reduced, the
ultimate stress is decreased and the ability for large elongation at yield limit is lost.




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 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

5.2       Effect of Corrosion on the Bars’ Physical Properties
         The effect of corrosion on steel bars physical properties was studied including the
 mass and rib loss for each type of steel at different degrees of corrosion. The mass loss was
 obtained as the difference between the mass of the corroded bar, after removal of the loose
 corrosion products, and its mass before corrosion. The ribs height were measured after the
 corrosion took place, and the rib profile loss was obtained as the difference between the rib
 height of the corroded bar and its height before corrosion. Tables 3 and 4 show the mass and
 rib profile loss of steel bars at different corrosion stages respectively.

                 Table 3: Mass loss of steel bars at different degrees of corrosion

                                                  Mass loss as a percentage of zero corrosion
      Concrete                                                     mass (%)
                           Steel Type
       Type
                                                 Pre-cracking   Cracking    Severe Corrosion
                          Plain (St37)                0.8           2.6               3.5
      30MPa,
                   Deformed Uncoated (St60)           1.2           2.9               4.2
      w/c=0.32
                    Deformed Coated (St60)            1.6           2.4               3.6
                          Plain (St37)                0.4           0.6               2.9
      44MPa,
                   Deformed Uncoated (St60)            1            1.2               3.6
      w/c=0.32
                    Deformed Coated (St60)             1            1.1               3.3
                          Plain (St37)                0.4           1.2               2.2
      60MPa,
                   Deformed Uncoated (St60)           0.5           1.7               2.9
      w/c=0.32
                    Deformed Coated (St60)            0.4           1.7               2.6
                          Plain (St37)                0.6           2.6               3.7
      44MPa,
                   Deformed Uncoated (St60)           0.4           1.6               4.2
      w/c=0.52
                    Deformed Coated (St60)            0.5           2.9               4.1


         From Table 3, the un-coated deformed bars have the greatest mass loss while the plain
 bars have the least mass loss in all concrete mixes. The epoxy coated bars have less mass loss
 compared to the uncoated deformed bars which proves the efficiency of the epoxy coating in
 corrosion protection. The epoxy coating is well known for its good protection for steel bars
 against corrosion. The epoxy coating in this research was applied to the whole length of the
 bar but its ends were left un-coated as practiced in the construction field. Therefore the
 corrosion is concentrated in the uncoated end leading to specific core rupture across the
 diameter. Figs. 3 and 4 show the difference in crack propagation between the un-coated and
 epoxy coated deformed bars respectively. The comparison between the un-coated deformed
 bars and plain bars, showed that the plain bars have less mass loss than the deformed ones.
 This can be related to the high surface area of deformed bars. The plain bars had also less
 mass loss than the deformed coated ones. The plain bars are steel 37, while the deformed bars
 are steel 60, therefore the difference in corrosion rate can be attributed to the difference in

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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

chemical composition between the two types of steel, showing that steel 37 is more resistant
to corrosion than steel 60. The uncoated bars also showed uniform corrosion along the total
bar length, while the epoxy coated bars showed concentrated corrosion at un-coated end
leading to localized stresses at the bottom third of the concrete cylinder.

        Table 4: Rib and diameter loss of steel bars at different degrees of corrosion

                                             Rib profile loss as a percentage of zero corrosion
 Concrete                                                      rib height (%)
                       Steel Type
  Type                                                                              Severe
                                             Pre-cracking        Cracking
                                                                                  Corrosion
                Plain St37(diameter loss)*         8.7            16.1              18.8
  30MPa,
                Deformed Uncoated St60             69.2          98.56            (131.2)**
  w/c=0.32
                 Deformed Coated St60              81.2           92.1            (122.1)**
               Plain ST37 (diameter loss)*         6.1             7.5              17.1
  44MPa,
                Deformed Uncoated St60             38.7           69.6            (121.9)**
  w/c=0.32
                 Deformed Coated St60              43.6            66             (116.9)**
               Plain ST37 (diameter loss)*          6             11.1              14.7
  60MPa,
                Deformed Uncoated St60             47.2           84.7            (108.4)**
  w/c=0.32
                 Deformed Coated St60              40.8           83.6            (103.9)**
               Plain ST37 (diameter loss)*         7.6             16               19.1
  44MPa,
                Deformed Uncoated St60             63.9           81.8            (132.1)**
  w/c=0.52
                 Deformed Coated St60              63.1           94.2            (130.9)**


*For plain bars with no ribs, the percentages were calculated from the total cross-sectional
area of the bar.
**Results exceeding the 100% indicate that the corrosion causes total loss in the ribs and
extends to the inner bar diameter.




   Figure 6 Crack propagation in concrete          Figure 7 Crack propagation in concrete
     cylinders reinforced with uncoated            cylinders reinforced with epoxy coated
               defromed bars                                    defromed bars




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     (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

            Concerning the rib height loss, Table 4 indicates that un-coated deformed bars exhibit
     the maximum rib loss when compared to the plain and epoxy coated deformed bars. From the
     previous two tables, it can be concluded that the rate of corrosion increases in case of un-
     coated deformed bars. This can be attributed to the greater surface area of deformed bars
     compared to the plain bars and their greater conductivity when compared to the epoxy coated
     bars, where the epoxy acts as a barrier to chloride penetration.

6.          CONCLUSION

             The purpose of this research is to study the effect corrosion on reinforced concrete
     structures especially those near the sea side and structures located in salt-laden environments.
     The influence of corrosion on the mechanical and physical properties of steel bars was
     investigated. This paper includes results from experimental program performed with four
     concrete mixes and three types of steel bars. All concrete mixes were reinforced by three
     different carbon steel types: Plain, deformed, and epoxy-coated deformed bars. The
     experimental results of steel bars tensile tests, showed that all bars demonstrate large ductility
     at yielding regions before corrosion. However, these regions starts to disappear as the
     corrosion propagated from pre-cracking to severe corrosion. At advanced corrosion stages;
     corrosion reduces the cross-sectional area of steel bars, and affect the height of ribs of
     deformed bars. Corrosion causes high loss of steel ductility, large reduction of yield and
     ultimate stresses. When combined with reduction of effective cross-sectional area of steel,
     corrosion leads to serious deterioration of load carrying of reinforced concrete members.

     REFERENCES

      [1] Hassan A.H., Sanad A.M., and Moussa M.A., Environment Impact on Sea Front
          Reinforced Concrete Structures in Egypt, Global Climate Change, Biodiversity and
          Sustainability, April 2013.
      [2] ASTM STP 1065, "Corrosion Rates of Steel in Concrete," ISBN13: 978-0-8031-1458-
          6, 1990.
      [3] Bertolini, L., (2004), “Corrosion of Steel in Concrete,” ISBN: 3-527-30800-8.
      [4] Sanad A.M., Hassan A.H. and Moussa M.A., Finite Element Modeling of Steel
          Corrosion in Reinforced Concrete Cylinders, The 3rd International Conference on
          Advanced Engineering Materials and Technology, May 2013
      [5] Assem Adel Abdel Aal Hassan, (2003), "Bond of Reinforcement in Concrete with
          Different Types of Corroded Bars" Theses and dissertations, Ryerson University.
      [6] Siddhant Datta , B.M. Nagabhushana and R. Harikrishna, “A New Nano-Ceria
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